1. Field of the Invention
The invention relates to apparatus and a method for use therein for converting a relatively high resolution halftone bit-mapped monochromatic document, such as illustratively a halftone separation and which exists in a CDPF print file, into a relatively low resolution continuous tone grey scale document which, when the latter is applied to a display screen of a video monitor having grey scale capability, would provide a readable displayed page, i.e. a "preview", that approximately depicts how the halftone separation would appear when printed.
2. Description of the Prior Art
Although electronic communication is becoming rather ubiquitous, currently the printed page is still the predominant form of communication. A printed page contains text, graphics and/or images.
In the graphics arts industry, a page that will be mass reproduced is commonly referred to as an artwork. Traditionally, artworks were produced using manual document creation and page composition processes. Unfortunately, such manual processes proved to be tedious and costly. As such, electronic systems that create images, graphics and text and provide electronic page composition capabilities are seeing increasing use. With these electronic systems, an artwork, owing to its inclusion of graphics and/or images, is often stored in bit-mapped form for reproduction on a raster based output device. For black and white artworks, each location in the bit-map contains a binary value that specifies whether the pel (single bit value) situated at that location in the artwork is either white or black. For grey scale or color artworks, each location in the bit-map contains a multi-bit pixel value that respectively corresponds to the particular color or monochromatic shade that exists at that location in the artwork. Now, even though an artwork can be processed and stored electronically, a paper reproduction of that artwork into a printed page will ultimately need to be made, particularly if that page will form part of a publication, such as a magazine.
Currently, printing presses of one form or another are generally used to provide accurate mass reproductions of a color artwork. To avoid the need to use a differently colored ink in the press to print each different color in the artwork, current printing techniques rely on the fact that any color can be obtained as a linear combination of four primary subtractive colors: cyan, yellow, magenta and black. As such, to print a multi-colored artwork, four printed monochromatic images, each made from a corresponding primary colored ink, are successively made in separate printing passes on a common sheet of printing stock and overlaid on that sheet with proper registration to yield a single multi-colored page that, when viewed by an observer, accurately replicates the tonal colorations of a given artwork.
In particular, a colored artwork that is to be printed is first separated, typically by optical filtering and photographic processes, into four primary color continuous tone ("contone") separations. Each separation is essentially a monochromatic two-dimensional depiction on a transparent medium of the color information for only one of the primary colors in the artwork. As such, a different separation exists for cyan, yellow, magenta or black. However, only one separation (typically black) would be used for single color grey-scale artworks. Unfortunately, printing presses are not able to apply a differential amount of a colored ink to any one location in a page. As such, a printing press can not directly print a contone separation. To surmount this obstacle, the art teaches the use of halftone separations. A halftone separation is formed through screening a contone separation. With no tonal variations, each halftone separation contains a regularly spaced two-dimensional pattern of relatively small monochromatic dots with a resolution in most graphic arts applications of at least 85 dots/inch (approximately 34 dots/centimeter). Such a regular dot pattern has a relatively high spatial frequency. As a result of screening a contone separation that has tonal variations, the tonal variations in each location in the separation of the artwork change the spacing between adjacent dots located in corresponding locations in a halftone separation thereby spatially modulating the underlying regular halftone dot pattern for that color. Hence, when the halftone separation is viewed by a human eye, the modulated pattern is integrated by the eye to yield the corresponding tonal variations. Once the four halftone separations are made, these separations are transferred to separate printing plates which, in turn, are subsequently used to print four halftone separation patterns on common sheets of printing stock with proper registration. When the resulting printed sheet is then viewed by an observer, an accurate depiction of the desired colored artwork results from the spatial interaction of the four overlaid primary colored halftone separation patterns.
Unfortunately, when the individual halftone separation patterns are overlaid, the dots in each of these patterns interact to frequently form objectionable low frequency beat or interference patterns that appear as a repeating rosette pattern in some region of the printed page. This beat pattern is commonly referred to as a Moire pattern (hereinafter referred to as Moire). If the frequency of the beat pattern is sufficiently low in this region, then the associated Moire is very visible, quite unsightly and highly objectionable to an observer.
With this in mind, those skilled in the art readily realize that, to reduce Moire, each separation needs to be screened at a different angle with respect to a common axis. In this regard, different preset screen angles are often used to generate corresponding halftone separations of an artwork, with the angles commonly being 45, 75, 90 and 105 degrees for black, magenta, yellow and cyan, respectively. Occasionally, these screen angles may need to be appropriately varied to a different value to shift the beat frequency to a higher value where the resulting Moire is less visible to an observer.
Unfortunately, the amount of Moire that actually exists in any printed color page is often not known until the halftone separation patterns for that page are actually overlaid to yield a so-called "proof". Sometimes, the Moire is not detectable until later in the printing process when printing plates are made of the halftone separation patterns and an actual test sheet, i.e. a so-called "press sheet", is actually printed from these plates. Moreover, the existence of various other undesirable artifacts in a halftone separation, such as spots, streaks or the like, and/or excessive dot gain affects that may need to be corrected are also frequently not known until the press sheet is actually made.
As a result of these undesirable affects, an iterative trial and error process is often undertaken by a color technician to generate a proof followed by a press sheet such that each of these affects, if they exist, can be seen and appropriate adjustments made in the printing process, e.g. rotating the screen and/or changing the separations to eliminate an artifact and/or changing the dot size, to ultimately generate an acceptable press sheet that accurately reproduces the colorations in a given artwork. However, due to the inherent variability in producing an acceptable set of halftone separations, this process can be tedious, inordinately time consuming and hence expensive.
In an effort to reduce the time required and expense associated with manual color reproduction processes, the art has turned away from use of these manual processes in high volume graphic art applications to the use of electronic page creation and composition systems, as noted above. These systems convert contone separations into electronic (often digital) form, electronically change screen angles and compensate dot size for expected dot gain, and electronically produce appropriate halftone separations thereby eliminating the need to photographically generate the separation transparencies and the proof. Such systems aim to produce accurate halftoned color separations at an increased throughput and at a lower cost than that heretofore possible with traditional manual color reproduction processes.
One component of an electronic image creation and composition system is a high resolution bit-mapped printer that is capable of directly printing a halftone separation. Such a printer which is particularly advantageous to print halftone separations is the IBM Model 4250 electro-erosion printer (IBM is a registered trademark of the International Business Machines Corporation in Armonk, N.Y.). This printer utilizes a special printing media formed of a thin aluminum sheet overlaying either a dark contrast layer or a transparent undercoat. During printing, the printer produces each darkened pel in a printed page on this printing media by selectively vaporizing (electro-eroding) the aluminum film which is situated at that pel location on the print media. If printing media with a dark contrast layer is used, then this media, when eroded, could serve as a camera ready master for a halftone separation. Alternatively, if printing media with a transparent undercoat is used, then this media, when eroded, could serve as either a camera ready negative or short run printing plate. Through this printing process, this printer is able to provide an all points addressable resolution of 600 dots (pels) per inch (approximately 236 dots/centimeter) which provides a printed halftone separation that is able to clearly depict rather fine detail. Such a printing density is approximately 4-6 times the current density of 60,000 to 90,000 dots/square inch (approximately 9,300 to 13,950 dots/square centimeter) that is attainable with present xerographic and laser printing techniques and thus is capable of depicting finer detail than that possible with these techniques.
Because of the enormous number of separate pels (5100-by-6600 or 33.66 million) that could comprise an electro-erosion print file for illustratively an 81/2 by 11 inch (approximately 22-by-28 centimeter) page, the printer would require a significant amount of time to produce a printed page, particularly if that image contains mostly dark areas. Inasmuch as generating an acceptable set of halftone separations often requires generating a set of separations, detecting abnormalities therein and varying separation parameters and then iteratively repeating this process until acceptable separations are produced, significant time would be expended if every separation that was generated during this iterative process was printed with the electro-erosion printer.
As such, to markedly decrease the turnaround time needed to form an acceptable set of halftone separations using an electro-erosion printer as part of an electronic page creation and composition system, it would be particularly advantageous to display a readable pattern, i.e. a "preview", on a video monitor that approximately depicts how a halftone separation pattern would appear when printed in lieu of actually printing that separation--provided that the time required to generate such a display would be significantly faster than the time required by the printer to actually print the separation. Through use of such a "preview" image, a color technician could, on an interactive basis, display a "preview" of a halftone separation pattern, determine if abnormalities exist in the "preview", make appropriate corrections to the separation parameters, e.g. vary screen angle or the like and/or dot size, electronically generate a new separation and obtain a "preview" of its halftone pattern to verify the accuracy of the new separation all in a relatively short interval of time. By employing such a "preview", the electro-erosion printer would only be used to generate a halftone separation after the "preview" indicates that the halftone pattern produced thereby is likely to be satisfactory. This, in turn, would likely and advantageously eliminate a significant number of printing operations as well as a significant portion of the tedium, time and expense associated with generating an acceptable set of color separations by electronic page creation and composition systems known in the art.
Unfortunately, generating a suitable "preview" for display on a video monitor has proven to be difficult. In particular, the resolution inherent in a bit-map produced for an electro-erosion printer (33.66 million single bit pels) is substantially larger than that provided by even a high resolution video display. Illustratively, a suitable high resolution display that possesses the capability of displaying a single color with at least 37 different shades is the IBM model PS/2 8514 video display. Such a display provides a resolution of 1024 horizontal pixels by 768 lines for a total display of 804,864 separate multi-bit (eight bit) pixels, with each pixel taking on any one of 64 different shades of a selected color. Unfortunately, while a video monitor produces separate dots, the spacing between adjacent dots (the so-called "writing pitch") is fixed, as contrasted with the variable spacing inherent in a halftone separation. The monitor only possesses the capability to display a fixed size dot or not and change the intensity of that dot, i.e. its shading.
Hence, not only must the speed of producing the "preview" be sufficiently fast and the resolution of the bit-mapped halftone separation pattern be converted to a resolution suitable for display on a video monitor, but also the spacing variations inherent in a halftone separation pattern must be properly converted to shading variations and the resulting displayed page must be sufficiently readable such that it is able to depict, to an observer, various abnormalities that could occur in the halftone separation pattern.
With these requirements in mind, none of the methods, commonly referred to as "anti-aliasing", known in the art for producing a continuous tone displayed image at a relatively low resolution from a much higher resolution bit-mapped halftoned separation pattern has proven to be satisfactory for use in generating such a "preview" image. For example, U.S. Pat. No. 4,630,125 (issued to P. Roetling on Dec. 16, 1986) discloses a method for unscreening a stored digital halftone image. Inasmuch as this method relies on processing both each pel in the screened image and also a neighborhood of surrounding pels, this method requires an inordinate amount of execution time. Moreover, this method does not address how the resolution of the bit-mapped halftone separation pattern would be converted to an image at a lower resolution suitable for display on a video monitor. U.S. Pat. No. 4,533,942 (issued to W. Gall et al on Aug. 6, 1985) addresses a method for reproducing a contone image with a coarser resolution for display on a video monitor than that at which the image was scanned. Unfortunately, this patent, which relies on forming a weighted sum of a field of fine resolution pixel values to form a corresponding single low resolution pixel value for display, does not address how a bit-map of single bit pel values would be converted to corresponding multi-bit grey scale pixel values.
Therefore, a need exists in the art for apparatus and particularly for a method implemented therein that is sufficiently fast for generating a "preview" image, at a relatively low resolution suitable for display on a video monitor and with appropriate shading variations, that approximately depicts how a higher resolution bit-mapped monochromatic pattern would appear when printed. By incorporating such a method into an electronic page creation and composition system, the time needed to generate an acceptable set of halftone color separations can be significantly reduced. This, in turn, can advantageously result in a concomitant increase in the throughput of high quality halftone separations than that which can be provided by such electronic systems known in the art. Moreover, use of such a method would advantageously allow an electro-erosion printer to be used in such a system for use in directly printing halftone separations that can depict finer detail than that heretofore possible with other printing techniques and without adversely affecting, to any appreciable extent--if at all, the turnaround time of the system.